These odd keys (such as G#) usually turn up in modulations. For example, a piece in C# might modulate to the V key, which would be G#. Using the G# signature maintains the true harmonic relationship with the home key -- A-flat would not. C# Major is a popular key for piano concertos (sounds great on the 88) and that type of modulation is common, even in folk music. I'm not saying I like G#, but it happens.
Because of the way the piano is tuned, each key/mode has a unique sonority, which composers are very well aware of. Have you noticed, as soon as you get beyond Lesson Three, there is not much piano music written in C major. That's because it sounds pretty crummy on the acoustic piano. G is much better.
Anyway, my favorite keys on the piano for general playing are D major and G minor, because of the sonorities and ease of fingering. I play a 5-string banjo (G tuning) and I actually like the sound better when capo'd on the first fret. Yikes, now we are back to G#, but let's call it Ab, thank you!
Your good singing keys depend a lot on your range. If you have two octaves, you can sing in any key. My two octave days are fading into the mists of time, but I can count on from C to F. Now, most singers have figured out there are two kinds of folk songs - those that range roughly from doh-to-doh, and the others from sol-to-sol. So in my case I would use D for the first kind, and G or A for the second kind. Or again, A flat.
== Johnny in Oklahoma City